Molybdenum oxide film resistor and the method of making same



3 474,007 MOLYBDENUM OXIDE FILM RESISTOR AND THE METHOD OF MAKING SAME .lohn Cotter Hogan, Cincinnati, Ohio, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Nov. 10, 1966, Ser. No. 594,318 lint. Cl. C231) 9/00, /00 US. Cl. 204-14 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a stable film resistor comprising an interference layer of a molybdenum oxide on a substrate.

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

This invention relates to film resistors including a layer of molybdenum oxide and to a method for making same.

More particularly, the present invention relates to film resistors of high stability which evidence favorable electrical resistance properties for use as a capacitor, as an electrolytic resistance, or as a dry resistance.

In order for molybdenum oxide to function as a film resistor, a method must be available which allows formation of an adherent, stable, continuous film of a molybdenum oxide on a substrate. Suitable substrate materials may comprise molybdenum metal or alloys containing molybdenum; metals other than, and alloys which do not contain molybdenum; or, non-conductors such as glasses or glazed ceramics, etc. In those cases where the substrate does not contain molybdenum, the substrate must first be coated with molybdenum. Thus, for example, Where the substrate is conducting, a layer of molybdenum can be chemically or electrically plated onto the surface of the substrate. Where the non-molybdenum containing substrate is a non-conductor, a layer of molybdenum can be applied by cathode sputtering techniques.

Assuming then the existence of a molybdenum-containing surface, this invention provides a film resistor containing a molybdenum oxide obtained by anodic oxidation of a molybdenum surface in a non-aqueous solvent containing an electrochemically oxidizing compound. More specifically, the process of this invention comprises connecting molybdenum as the anode in an electrolytic cell, contacting said anode and a cathode in a non-aqueous solution having a dielectric constant in the range 5 to 20 in which the electrolytically formed molybdenum oxide is substantially insoluble, and then impressing a potential of -100 volts between said cathode and anode for a selected time interval to control the thickness of the resultant oxide film. A preferred anodizing composition is a soluton of an alkali metal nitrate such as potassium or sodium nitrate dissolved in acetone. The solvent must meet two requirements: It must first have a dielectric constant above 5 in order that passage of sufficient current between the electrodes is effected to oxidize the surface of the molybdenum. In addition, the solvent must not dissolve the molybdenum oxide film formed by the anodic oxidation process. The use of solvents more polar than acetone, for example, water or ethyl alcohol results in dissolution of the film and pitting of the molybdenum anode. On the other hand, such solvents as acetone, acetic acid, acetic acid+acetic anhydride, also ketones, aliphatic acids, aliphatic alcohols having more than 3 carbon atoms, and mixtures thereof will be found suitable in that upon the application of an impressed potential between the electrode sufiicient conductance is developed to create a current in the electrolytic circuit and form the desired anodized molybdenum oxide film. By the term electro- 'ited States Patent 0 "ice chemically oxidizing is meant a compound which under an electrical potential can function as an oxidizing agent in solution either directly or through some intermediate chemical action. Examples of electrochemically oxidizing salts which can be used in combination with the previously defined solvent are NaNO KNO NH,NO

NaNO +Pb 2 0 3) 3)z 3)2, a s)2, and NaNO +KNO The thickness of the resultant molybdenum oxide film will be determined by the time at applied potential. As the molybdenum oxide film is formed, the current measured between the electrodes drops sharply at constant anodic potential and with increasing film thickness. For example, at an applied potential of from 10-70 volts with an initial current density of from 20-100 milliamperes/ cm. the molybdenum oxide film is formed as the current density drops to from 10400 microamperes/cmF.

'In a non-solvent, the anodized molybdenum oxide film forms a typical interference film (i.e., appears clear, transparent and varying in color depending on thickness) whereas the blue molybdenum oxide film formed in a solvent (i.e., water containing bath) is not of an interference type, is opaque, porous, and reflects the color of the M00 compound rather than a color indicative of the thickness of an interference film. Thus, at some thicknesses the film formed in a non-solvent (molybdenum oxide) can be blue, yellow, green, brown, etc., whereas the film formed in a water containing solution is always blue.

While a molybdenum oxide film can form in a solution which functions as a molybdenum oxide solvent, its existence is transitory and is visible as a film only during passage of current. When there is no anodizing current the oxide will dissolve in the solvent.

A representative embodiment of the method of the present invention together with examples which demonstrate the utility of the molybdenum oxide film as an electric or electrolytic resisting element is described below.

EXAMPLE A number of thin sheets of molybdenum were anodized in an acetic solution saturated with potassium nitrate at room temperature. Using a Mo, Cu, or Fe cathode set up to face the molybdenum anode and separated by a distance of 1-10 cm., the molybdenum sheet was anodized at a potential of about 25-30 volts with an initial current density averaging 15 milliamperes/cm. which decreased to less than 20 microamperes/cm. as the molybdenum oxide film was formed. The anodized molybdenum sheet was removed from the bath, washed with acetone, and then dried with soft paper all at room temperature. The anodized film thickness was estimated by the interference color of the oxide film (light to dark blue) to be from 400-800 angstroms.

A number of anodized specimens were made in the manner described and then tested for their electrical or electrolytic resistance properties as follows:

(a) As a capacitor-The molybdenum oxide coated sheet was covered with a piece of thin porous paper saturated with acetic acid, and over this was placed a piece of copper foil. Thus, an electrolytic capacitor was formed. The capacitance in two such cases was 0.18 microfarad with a dissipation factor of 0.8 for surface area of approximately 1.5 cm. of oxide film. These fihns will also take up a charge; giving about microampere kick when discharged after charging at 30 volts. The charging process can be repeated any number of times; the rate of charging increasing each time after discharge. Capacitors can be formed from 10 to at least 40 volts with a breakdown voltage of over at least 30 volts.

(b) As an electrolytic diode-These same films show a difference in resistance in one direction as opposed to the other. For example, when the film is attached to the positive side of a resistance meter, the resistance measures from 10100 megohms at an applied potential of 1.2 volts, depending on the thickness of the film; whereas, in the opposite direction (with the film attached to the negative side of the resistance meter) the resistance is only 7,000 ohms, a factor diiference of between 1,000 and 10,000 to 1. When measurements were made at 30 volts a ratio of 1 megohm to 1,000 ohms was obtained with oxide surface areas of about 3 cm.

(c) As a dry resistance-The resistance of the oxide film without electrolytic connection was found to be between 200,000 and 5 megohms for point contact of films formed at 3040 volts. These films had an approximate thickness of 100-500 angstroms.

What is claimed is:

1. A method of forming a continuous, adherent layer of molybdenum oxide exhibiting electrical and electrolytic resistance properties which comprises anodizing a References Cited UNITED STATES PATENTS 2,351,639 6/1944 Schweikher 20456 2,909,470 10/1959 Schmidt 204-l4 2,941,929 6/1960 Lilienfeld 204-l4 3,386,896 6/1968 Finne 204-38 JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner 

